(1) Field of the Invention
This invention relates to a composite fibrous sheet, especially to a napped composite fibrous sheet which has good mechanical properties while also having the desired softness.
(2) Description of the Prior Art
The use of "artificial leather" produced by applying a viscoelastic substance to a fibrous sheet has been ever-increasing in recent years.
Uses include not only shoes, bags, upholstery and the like but also clothing. Typical requirements for artificial leather are that it must be soft and, at the same time, must have good mechanical properties such as high abrasion resistance and high tear strength. It must also have the desired surface appearance.
Various attempts have been made to satisfy these requirements, as exemplified by the following methods. British Pat. No. 1,300,268 discloses a typical method which comprises applying a rubber-like viscoelastic substance to a fibrous sheet consisting of specific fibers, such as "islands-in-a-sea" type fibers or filaments, and then removing part of the specific fibers such as the sea component in order to obtain a sheet material having a soft and pliable hand.
The method typified by U.S. Pat. No. 3,706,613 comprises applying first a soluble binder such as polyvinyl alcohol to a sheet consisting of specific fibers such as "islands-in-a-sea" type composite fibers or filaments, then removing part of the specific fibers such as, for example, the sea component to generate superfine fibers, applying a viscoelastic substance such as polyurethane or the like and thereafter removing the soluble binder. Various lubricants or softeners may be added to the binder or to the product to soften the product.
These methods have merit, but they also have the following critical problems. The sheet obtained by first applying a rubber-like viscoelastic substance and then removing one component of the specific fibers, as typified by the abovementioned British Pat. No. 1,300,268, produces a sheet having a soft hand. However, the sheet has low tensile strength due to gaps generated between the superfine fibers and the rubber-like viscoelastic substance. When the sheet is raised to produce a napped surface effect, the nap easily peels off, and yielding a serious drawback. Further, since the solvent for the viscoelastic substance is, in most cases, also a solvent for the fiber-forming components, application of the viscoelastic substance becomes extremely unstable and continuous production of uniform product is difficult.
In the method exemplified by U.S. Pat. No. 3,706,613, there is a high probability of the superfine fibers and the rubber-like viscoelastic substance being bonded directly to one another. This produces a product with a secure nap, but the bonded parts between the superfine fibers and the rubber-like viscoelastic substance are relatively large, yielding a product with a rubber-like hand which is undesirable in clothing.
Attempts have been made to improve the mechanical properties and hand of the sheet material by carrying out the following steps:
(a) forming a thinnable fiber aggregated sheet;
(b) applying a dispersion and/or solution of elastic polymer (A) to the fiber aggregated sheet before or after the fibers of the fiber aggregated sheet are temporarily fixed, and then coagulating the polymer (A);
(c) temporarily fixing the fibers if they are impregnated with the polymer (A) before they are temporarily fixed or if temporary fixing is not sufficient;
(d) smoothing the surface of the sheet fibers using a press or a calender roll if the surface is not sufficiently flat;
(e) impregnating the fiber aggregated sheet with a dispersion and/or solution of a polymer (B) consisting principally of an elastic polymer and coagulating the polymer (B);
(f) removing the temporary fixing paste when the temporary fixing paste is used for temporary fixing;
(g) dissolving or decomposing and removing part or the whole of the matrix components of the fibers, thereby changing the fibers into bundles consisting of a number of thin fibers of small denier;
(h) raising one or both surfaces of the dried sheet to form napped surface or surfaces; and
(i) applying other necessary finish treatments such as dying, softening and the like.
This method is not completely satisfactory. Due to a lack of bonding between the rubber-like viscoelastic substance and the fibers forming the sheet, the tensile strength is not improved and the nap easily peels off the napped sheet. When a lubricant is added to the binder or applied to the product, particularly in apparel applications, a softening effect is achieved. However, this lubricant easily bleeds out and is removed during cleaning, leaving an uncomfortable garment with a harsh hand.